These four things are very useful to know:
Fixed ladder : Spawns a very tall ladder.
Drawbridge : Spawns a very long platform.
Chain : Spawns a bit longer rope-like item than the rope.
Brother/Priest/Exorcist (and give him any weapon) : An almost invincible attacker, only beatable by vampire.
And these things is good to know it's there:
Hair dryer : Put it in the water to instantly kill any water creature in a pretty large raidus. (also destroys starite
Geyser/hose : One of the best way to put out fires.
Mind control device : Can turn the fierce creatures to help you.
Shrink ray : Very useful if used with boulders or other large things that stand in your way.
Magic carpet : This thing is rather buggy, it lets you slip through small gaps in some levels.